Ligand having agonistic activity to mutated receptor

ABSTRACT

The present inventors used antibody engineering techniques to prepare functional antibodies that correspond to individual mutations in causative genes of diseases, and discovered that such antibodies enable the treatment of the diseases. Specifically, the inventors succeeded in preparing ligands, particularly minibodies, which have agonistic activity to receptors that have almost completely lost responsiveness to their natural ligands because of gene mutations (for example, a thrombopoietin (TPO) receptor whose reactivity to TPO has been markedly impaired), and which can transduce signals by interacting with these mutant receptors at levels comparable to normal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of International Application No. PCT/JP2004/003334, filed Mar. 12, 2004, which claims the benefit of Japanese Patent Application Ser. No. 2003-067832, filed on Mar. 13, 2003. The contents of both applications are hereby incorporated by reference in their entireties.

TECHNICAL FIELD

The present invention relates to ligands having agonistic activity to mutant receptors, and pharmaceutical compositions comprising the ligands as active ingredients.

BACKGROUND ART

In recent years, the causative genes of various diseases have been identified in quick succession, and a variety of therapeutic methods for such diseases have been studied and established. Of these methods, the most intensively studied are mostly therapeutic methods for complementing enzyme gene deficiencies. It has been reported that enzyme replacement therapy using “Cerezyme” (Genzyme) is effective for patients with Gaucher's disease, in which β-glucocerebrosidase is deficient, and that enzyme replacement therapy using “Aldurazyme” (Genzyme) is effective for patients with mucopolysaccharidosis, in which α-L-iduronidase is deficient. Previously attempted gene therapies include introducing the adenosine deaminase (ADA) gene to patients with ADA deficiency, and introducing the coagulation factor IX gene to patients with hemophilia B. In addition to enzyme deficiencies, a large number of genetic diseases are known, such as genetic diseases of cytokines and their receptors. Some patients with type II diabetes mellitus, which accounts for approximately 90% of diabetes mellitus cases, have been reported to have insulin receptor deletions or mutations. Such deletions and mutations are assumed to cause the disease. Furthermore, some patients with thrombocytopenia have been reported to have thrombopoietin receptor deletions and mutations, and the failure of TPO signaling can be thought to cause the disease. To date, no fundamental therapeutic methods have been available for such genetic diseases, and the establishment of such therapeutic methods is expected.

Congenital amegakaryocytic thrombocytopenia (CAMT) is a rare disorder that causes thrombocytopenia in infancy and pancytopenia in later childhood. It has been revealed that TPO, a thrombopoietic growth factor, is present in CAMT patients at a high concentration in sera, but that platelets and hematopoietic precursor cells lack TPO responsiveness. Most of these patients have been found to carry point mutations in their thrombopoietin receptor (c-MPL) gene. It has also been reported that such mutations result in frame shift or insertion of a termination codon, leading to patients who have a total loss of thrombopoietin receptor function and patients who have a homozygous or heterozygous amino acid substitution in the extracellular domain of the receptor (see Non-patent Document 1). Bone marrow transplant is the only currently available therapeutic method for treating such patients.

[Non-Patent Document 1]

Matthias Ballmaier, Manuela Germeshausen, Harald Schulze, Klara Cherkaoui, Sabine Lang, Annika Gaudig, Stephanie Krukemeier, Martin Eilers, Gabriele Strausz, and Karl Welte, “BLOOD”, vol. 97, No. 1, pp. 139 (Jan. 1, 2001).

DISCLOSURE OF THE INVENTION

The present invention was achieved in view of the circumstances described above. An objective of the present invention is to provide ligands having agonistic activity to mutant receptors.

The present inventors studied intensively to achieve the objective described above. By using antibody engineering techniques, the inventors prepared functional antibodies corresponding to each of the mutations in the causative genes of the diseases described above, discovering that these antibodies were useful to treat such diseases. Specifically, the inventors succeeded in preparing minibodies, each of which has agonistic activity to a mutant receptor that has almost completely lost responsiveness to its natural ligand due to gene mutation (for example, a thrombopoietin (TPO) receptor whose responsiveness to TPO is mostly lost), and which can transduce signals at levels comparable to normal levels when reacted with such a mutant receptor.

Diseases caused by gene mutations can be treated by using such antibodies or modified products thereof. Alternatively, such diseases can be treated by gene therapy using genes capable of expressing the antibodies or modified products thereof.

CAMT patients, who were previously difficult to treat, can now be treated using ligands having agonistic activity, such as the antibodies of the present invention or the genes encoding them.

In addition, the methods of the present invention are applicable to various other diseases caused by mutations in genes encoding receptors on cell membranes. Thus, the present invention can provide new therapeutic methods for these diseases.

Specifically, the present invention relates to ligands having agonistic activity to mutant receptors, more specifically provides:

[1] a ligand having agonistic activity to a mutant receptor;

[2] the ligand of [1], where the ligand is an antibody;

[3] the ligand of [1] or [2], where the ligand has greater agonistic activity to the mutant receptor than the natural ligand;

[4]. the ligand of any one of [1] to [3], where the mutant receptor is a receptor resulting from a mutation(s) in the amino acid sequence;

[5] the ligand of any one of [1] to [4], where the mutant receptor has lost responsiveness to the natural ligand;

[6] the ligand of any one of [1] to [5], where the mutant receptor causes a disease;

[7] the ligand of any one of [1] to [6], where the mutant receptor is a mutant thrombopoietin receptor;

[8] the ligand of [2], where the antibody is a minibody;

[9] the ligand of [8], where the minibody is a diabody;

[10] the ligand of [8], where the minibody is sc(Fv)2;

[11] a method for transducing a signal to a mutant receptor by binding a ligand;

[12] the method of [11], where the ligand is an antibody;

[13] the method of [11] or [12], where the mutant receptor results from an amino acid mutation(s);

[14] the method of any one of [11] to [13], where the mutant receptor has lost responsiveness to the natural ligand;

[15] the method of any one of [11] to [14], where the mutant receptor is associated with disease onset;

[16] the method of any one of [11] to [15], where the mutant receptor is a mutant thrombopoietin receptor;

[17] a method for treating a disease caused by a mutant receptor, by binding a ligand to the mutant receptor;

[18] the method of [17], where the ligand is an antibody;

[19] a method of screening for a ligand having agonistic activity to a mutant receptor, where the method comprises the steps of,

(a) contacting a test substance with the mutant receptor,

(b) detecting a signal in the mutant receptor, and

(c) selecting a ligand having agonistic activity;

[20] a method of screening for a ligand having agonistic activity to a mutant receptor, where the method comprises the steps of,

(a) determining agonistic activity to a normal receptor,

(b) determining agonistic activity to the mutant receptor, and

(c) selecting a ligand having greater agonistic activity to the mutant receptor than the normal receptor;

[21] a method of screening for a ligand having agonistic activity to normal and mutant receptors, where the method comprises the steps of,

(a) determining agonistic activity to the normal receptor,

(b) determining agonistic activity to the mutant receptor, and

(c) selecting a ligand having agonistic activity to both normal and mutant receptors;

[22] the method of any one of [19] to [21], where the ligand is an antibody;

[23] a substance obtained by the method of any one of [19] to [22];

[24] a therapeutic agent for a disease caused by a mutant receptor, where the agent comprises a ligand of the mutant receptor;

[25] the therapeutic agent of [24], where the ligand is the ligand of any one of [1] to[9];

[26] the therapeutic agent of [24], where the ligand is an antibody;

[27] the therapeutic agent of any one of [24] to [26], where the mutant receptor results from an amino acid mutation(s);

[28] the therapeutic agent of any one of [24] to [27], where the mutant receptor has lost responsiveness to the natural ligand;

[29] the therapeutic agent of any one of [24] to [28], where the mutant receptor is a mutant thrombopoietin receptor; and,

[30] the therapeutic agent of any one of [24] to [29], where the disease is congenital amegakaryocytic thrombocytopenia.

The present invention provides ligands having agonistic activity to mutant receptors.

Mutant receptors of the present invention are usually receptors that exist at a frequency of less than 50%, preferably less than 20%, more preferably less than 10%, and even more preferably less than 1%. The frequency is generally calculated using randomly selected subjects. However, the frequency may vary depending on country, area, sex, and such. Therefore, the frequency may also be calculated, for example, within a selected country or area such as Japan, the United States, or Europe, or be calculated for one sex. When there are two or more mutation sites in a receptor, the frequency may be calculated for multiple mutation sites or for any one of the mutation sites. Mutant receptors are preferably evaluated using frequency, as described above. However, mutant receptors can also be evaluated, for example, by their signal transducing ability. Specifically, for example, when two different receptors are present, the one with stronger transducing signals upon natural ligand-binding may be used as a non-mutant receptor, and the other with weaker transducing signals may be used as a mutant receptor.

Preferred mutant receptors of the present invention include, but are not limited to, receptors resulting from amino acid mutations (receptors with mutated amino acid sequences); however, any type of mutation is acceptable, as long as the mutated receptor influences responsiveness to natural ligands, or the conformation, sugar chain structure, or spatial relationship or angles when a receptor exists as a multimer, and so on. Mutations in the amino acid sequence include amino acid substitutions, deletions, insertions, and additions. The receptors of the present invention have preferably lost responsiveness to the natural ligands.

Herein, “ligand” refers to a substance that specifically binds to a functional protein. The type of ligand is not limited. Such ligands include low-molecular-weight compounds, proteins, and peptides. In the present invention, functional proteins are preferably receptors. In the present invention, ligands preferably have agonistic activity. The present invention also provides methods for transducing signals to a mutant receptor by binding a ligand of the present invention. Such ligands for use in the methods of the present invention are preferably non-natural ligands, and not natural ligands.

In the present invention, it is preferred to target a mutant receptor whose responsiveness to a natural ligand is different from that of a non-mutant receptor. A “mutant receptor whose responsiveness to a natural ligand is different from that of a non-mutant receptor” refers to a mutant receptor that exhibits agonistic activity and signaling activity that differs from the activities of the non-mutant receptor when the mutant and non-mutant receptors bind to the same natural ligand under the same conditions. In general, agonistic activity and signaling activity in mutant receptors is impaired compared with non-mutant receptors (the mutants have lost their responsiveness to natural ligands).

The receptors include receptors belonging to the hematopoietic growth factor receptor family, the cytokine receptor family, the tyrosine kinase receptor family, the serine/threonine kinase receptor family, the TNF receptor family, the G protein-coupled receptor family, the GPI-anchored receptor family, the tyrosine phosphatase receptor family, the adhesion factor family, and the hormone receptor family. There are many documents that describe receptors belonging to these receptor families, and their characteristics; for example Cooke B A, King R J B, van der Molen H J Eds. New Comprehensive Biochemistry Vol.18B “Hormones and their Actions Part II” pp. 1-46 (1988) Elsevier Science Publishers BV, New York, USA; Patthy L. (1990) Cell, 61: 13-14; Ullrich A. et al. (1990) Cell, 61: 203-212; Massagul J. (1992) Cell, 69: 1067-1070; Miyajima A. et al. (1992) Annu. Rev. Immunol., 10: 295-331; Taga T. and Kishimoto T. (1992) FASEB J., 7: 3387-3396; Fantl W I. et al. (1993) Annu. Rev. Biochem., 62: 453-481; Smith C A., et al. (1994) Cell, 76: 959-962; Flower D R. (1999) Biochim. Biophys. Acta, 1422: 207-234; Cell Technology: supplementary vol. Handbook series “Handbook for Adhesion factors” M. Miyasaka Ed. (1994) Shujunnsha, Tokyo, Japan, and so on. Specific receptors belonging to the families listed above include: human and mouse erythropoietin (EPO) receptors, human and mouse granulocyte-colony stimulating factor (G-CSF) receptors, human and mouse thrombopoietin (TPO) receptors, human and mouse insulin receptors, human and mouse Flt-3 ligand receptors, human and mouse platelet-derived growth factor (PDGF) receptors, human and mouse interferon (IFN)-α and -β receptors, human and mouse leptin receptors, human and mouse growth hormone (GH) receptors, human and mouse interleukin (IL)-10 receptors, human and mouse insulin-like growth factor (IGF)-I receptors, human and mouse leukemia inhibitory factor (LIF) receptors, and human and mouse ciliary neurotrophic factor (CNTF) receptors (hEPOR: Simon, S. et al. (1990) Blood 76, 31-35; mEPOR: D'Andrea, AD. Et al. (1989) Cell 57, 277-285; hG-CSFR: Fukunaga, R. et al. (1990) Proc. Natl. Acad. Sci. USA. 87, 8702-8706; mG-CSFR: Fukunaga, R. et al. (1990) Cell 61, 341-350; hTPOR: Vigon, I. et al. (1992) 89, 5640-5644; mTPOR: Skoda, R C. Et al. (1993) 12, 2645-2653; hInsR: Ullrich, A. et al. (1985) Nature 313, 756-761; hFlt-3: Small, D. et al. (1994) Proc. Natl. Acad. Sci. USA. 91, 459-463; hPDGFR: Gronwald, R G K. Et al. (1988) Proc. Natl. acad. Sci. USA. 85, 3435-3439; hIFN α/β R: Uze, G. et al. (1990) Cell 60, 225-234, and Novick, D. et al. (1994) Cell 77, 391-400).

In one embodiment, the mutant receptors of the present invention comprise receptors associated with disease onset. The phrase “mutant receptors associated with disease onset” means that the loss of responsiveness to a natural ligand is part of the reason that disease onset is triggered. In the present invention, a mutant receptor is not necessarily the sole factor triggering disease onset, and may be a contributing factor. Many reports have been previously published describing the association of mutant receptors with disease onset; however, in addition to previously reported associations, associations of mutant receptors and disease onset can also be identified by methods of statistical analysis (for example, correlation analyses). Correlation analyses, also called “case control studies”, are well known to those skilled in the art (for example, Nishimura, Y., 1991, “Statistical analysis of polymorphisms”, Saishin Igaku, 46:909-923; Oka, A. et al., 1990, Hum. Mol. Genetics 8, 2165-2170; Ota, M. et al., 1999, Am. J. Hum. Genet. 64, 1406-1410; Ozawa, A. et al., 1999, Tissue Antigens 53, 263-268). For example, the correlation between a mutant receptor and a disease can be studied by determining the frequency of the mutant receptor in patients and in healthy subjects, and examining whether the patient population has a higher mutant receptor frequency. Typically, differences in the frequency are evaluated using the x-test. x is obtained by the equation x²=Σ(observed value−expected value)²/expected value. A p value is obtained from the x² value thus determined. Whether a mutant receptor correlates with a disease can be determined based on this p value. For example, when p<0.05, a mutant receptor is considered to correlate with a disease.

There are many reports on mutant receptors known to be involved in disease onset. Such mutant receptors specifically include: mutant thrombopoietin (TPO) receptors, mutant insulin receptors, mutant erythropoietin receptors, mutant growth hormone receptors, mutant common γ chain receptors (common receptor of IL-2, IL-4, IL-7, IL-15, and IL-21), mutant androgen receptors (Glutamine Repeats and Neurodegenerative Disease: Molecular Aspects (2001), 261-267, Oxford University press), mutant receptors for proopiomelanocortin (POMC) and melanocortin (Journal of Clinical Endocrinology and Metabolism (2001), 86(4), 1442-1446), mutant ryanodine receptors (Human Mutation (2000), 15(5), 410-417), mutant thyroid-stimulation hormone receptors (Trends in Endocrinology and Metabolism (1998), 9(4), 133-140), and mutant thyrotropin receptors (European Journal of Medical Research (1996), 1(10), 460-464). In the present invention, particularly preferred mutant receptors are mutant thrombopoietin receptors.

Herein, a “natural ligand” refers to a ligand in the body, and is preferably a ligand with the most influence on non-mutant receptor signaling. Normally, the natural ligands of the present invention do not comprise antibodies.

“Agonistic activity” refers to the activity of inducing a change in a certain physiological activity, caused by transducing signals into cells upon the binding of a ligand to a receptor. The physiological activity includes, but is not limited to, growth-promoting activity, survival activity, differentiation activity, transcription activity, membrane transport activity, binding activity, proteolytic activity, phosphorylation/dephosphorylation activity, oxidation/reduction activity, transfer activity, nucleolytic activity, and dehydration activity.

In the present invention, any detection indicator may be used to assay physiological activities, as long as it can measure quantitative and/or qualitative change. For example, it is possible to use cell-free assay indicators, cell-based assay indicators, tissue-based assay indicators, and in vivo assay indicators. Indicators that can be used in cell-free assays include enzymatic reactions and quantitative and/or qualitative changes in proteins, DNAs, or RNAs. Such enzymatic reactions include amino acid transfers, sugar transfers, dehydrations, dehydrogenations, and substrate cleavages. Alternatively, the followings can be used: protein phosphorylations, dephosphorylations, dimerizations, multimerizations, catabolisations, dissociations and such; and DNA or RNA amplifications, cleavages, and extensions. For example, protein phosphorylation downstream of a signal transduction pathway may be used as a detection indicator. As indicators in cell-based assays, changes to cell phenotype, for example, quantitative and/or qualitative changes in products, changes in proliferation activity, morphological changes, or changes in cellular properties can be used. Products include secretory proteins, surface antigens, intracellular proteins, and mRNAs. Morphological changes include changes in process formation and/or process number, changes in cell flatness, changes in the degree of elongation/horizontal to vertical ratio, changes in cell size, changes in intracellular structure, heterogeneity/homogeneity of cell populations, and changes in cell density. Such morphological alterations can be observed under a microscope. Cellular properties to be used include anchorage dependency, cytokine-dependent responsiveness, hormone dependency, drug resistance, cell motility, cell migration activity, pulsatility, and alteration in intracellular substances. Cell motility includes cell infiltration activity and cell migration activity. Changes in intracellular substances include changes in enzyme activity, mRNA levels, levels of intracellular signaling molecules such as Ca²⁺ and cAMP, and intracellular protein levels. Furthermore, changes in the cell proliferation activity induced by receptor stimulation can be used as an indicator. The indicators to be used in tissue-based assays include functional changes for the tissue in use. Indicators that can be used for in vivo assays include changes in tissue weight, changes in the blood system (for example, changes in the number of hemocytes, protein contents, or enzyme activities), changes in electrolyte levels, and changes in the circulatory system (for example, changes in blood pressure or heart rate).

The methods for measuring such detection indicators are not particularly limited. For example, luminescence, color development, fluorescence, radioactivity, fluorescence polarization values, surface plasmon resonance signals, time-resolved fluorescence, mass, absorption spectrums, light scattering, and fluorescence resonance energy transfers may be used. These measurement methods are known to those skilled in the art and may be appropriately selected depending on the purpose. For example, absorption spectra can be measured using a conventional photometer, plate reader, or such; luminescence can be measured with a luminometer or such; and fluorescence can be measured with a fluorometer or such. Mass can be determined with a mass spectrometer. Radioactivity can be determined with a measurement device such as a gamma counter, depending on the type of radiation. Fluorescence polarization values can be measured using BEACON (TaKaRa). Surface plasmon resonance signals can be obtained using BIACORE. Time-resolved fluorescence, fluorescence resonance energy transfer, or such can be measured with ARVO or such. Furthermore, a flow cytometer can also be used for measuring. In the present invention, it is possible to use a chimeric receptor comprising an extracellular domain of a mutant receptor and a cytoplasmic domain of another protein. For example, when the cytoplasmic domain of G-CSF receptor, EPO receptor, EGF receptor, or thrombopoietin receptor is used, the cell proliferation activity induced by stimulating the receptor can be used as a detection indicator. In assays using cell proliferation activity as a detection indicator, cell lines that die in the absence of ligands are preferably used to improve detection sensitivity. Cytokine-dependent cell lines are particularly preferred because they can be easily passaged. For example, it is possible to use CTLL-2 cells, which are an IL-2-dependent cell line, and 32D cells, FDC-P1 cells, and Ba/F3 cells, which are IL-3-dependent cell lines. These cell lines will die two or three day after the start of culture when a cytokine such as IL-2 or IL-3 that is essential for cell proliferation, is eliminated from the culture media. It is preferable to use FDC-P1 cells and Ba/F3 cells that express a chimeric receptor comprising the cytoplasmic domain of mouse G-CSF receptor.

The ligands of the present invention that have agonistic activity are not particularly limited, as long as they have agonistic activity to mutant receptors. Such ligands may have agonistic activity to both mutant and non-mutant receptors, or may only have agonistic activity to mutant receptors. When the ligands have agonistic activity to both mutant and non-mutant receptors, they may have greater agonistic activity to the non-mutant receptor, or greater agonistic activity to the mutant receptor. Alternatively, the ligands may have comparable agonistic activity to the non-mutant and mutant receptors. Nonetheless, when the chief purpose is to treat a disease caused by a mutant receptor, it is preferable to use a ligand with greater agonistic activity to the mutant receptor than the non-mutant receptor.

The antibodies of the present invention are not particularly limited, as long as they have agonistic activity, and mouse antibodies, rat antibodies, rabbit antibodies, sheep antibodies, camel antibodies, chimeric antibodies, humanized antibodies, human antibodies, and such can be appropriately used. Such antibodies may be polyclonal or monoclonal antibodies. Monoclonal antibodies are preferred because they can be stably produced as homogeneous antibodies. Both polyclonal and monoclonal antibodies can be prepared by methods known to those skilled in the art.

Hybridomas producing monoclonal antibodies can basically be prepared by the conventional methods described below. Specifically, immunization is carried out by a conventional immunization method, using a desired antigen or cells expressing the desired antigen as a sensitizing antigen. The prepared immunocytes are fused with known parental cells by a conventional cell fusion method. The fused cells are screened for monoclonal antibody-producing cells (hybridomas) by conventional screening methods.

The type of sensitizing antigen to be used is not limited. For example, a full-length protein of a receptor of interest, or a partial peptide thereof (for example, an extracellular domain) can be used. The antigens can be prepared by methods known to those skilled in the art; for example, according to methods using baculovirus (for example, see WO 98/46777). Hybridomas can be prepared, for example, by the method of Milstein et al. (Kohler, G. and Milstein, C., Methods Enzymol., 1981, 73, 3-46.). When an antigen has weak immunogenicity, the antigen may be conjugated with a large immunogenic molecule such as albumin, to achieve immunization. In addition, the present invention may use recombinant antibodies, produced by gene engineering. The genes encoding the antibodies are cloned from hybridomas, inserted into an appropriate vector, and then introduced into a host (see, e.g., Carl, A. K. Borrebaeck, James, W. Larrick, Therapeutic Monoclonal Antibodies, Published in the United Kingdom by Macmillan Publishers Ltd, 1990). Specifically, using a reverse transcriptase, cDNAs encoding the variable regions (V regions) of the antibodies are synthesized from the mRNAs of the hybridomas. DNAs encoding the variable regions of the antibodies of interest are obtained, and ligated with DNAs encoding desired constant regions (C regions) of the antibodies, and these constructs are inserted into expression vectors. Alternatively, DNAs encoding the variable regions of the antibodies may be inserted into expression vectors comprising the DNAs of the antibody C regions. Those cDNAs are inserted into expression vectors such that the genes are expressed under the regulation of an expression regulatory region, for example, an enhancer and promoter. Host cells are then transformed using the expression vectors, and the antibodies can be expressed. The epitopes on the molecules that are recognized by antibodies of the present invention are not limited to particular epitopes. The antibodies may recognize any epitope on the molecules. Typically, the antibodies recognize epitopes in the extracellular domain. In the present invention, recombinant antibodies artificially modified to reduce heterologous antigenicity against humans can be used. Examples of such recombinant antibodies include chimeric antibodies and humanized antibodies. These modified antibodies can be produced using known methods. A chimeric antibody comprises heavy chain and light chain variable regions of an antibody from a nonhuman mammal such as a mouse, and heavy chain and light chain constant regions of a human antibody. Such an antibody can be obtained by (1) ligating a DNA encoding a variable region of a mouse antibody to a DNA encoding a constant region of a human antibody; (2) inserting the resulting construct into an expression vector; and (3) introducing the vector into a host for production of the antibody. A humanized antibody, which is also called a reshaped human antibody, is obtained by transferring a complementarity determining region (CDR) of an antibody of a nonhuman mammal such as a mouse, to the CDR of a human antibody. Conventional genetic recombination techniques for the preparation of such antibodies are known. Specifically, a DNA sequence designed to ligate a CDR of a mouse antibody with the framework regions (FRs) of a human antibody is synthesized by PCR, using several oligonucleotides constructed to comprise overlapping portions at their ends. A humanized antibody can be obtained by (1) ligating the obtained DNA to a DNA that encodes a human antibody constant region; (2) inserting the resulting construct into an expression vector; and (3) introducing the vector into a host to produce the antibody (see European Patent Application No. EP 239,400, and International Patent Application No. WO 96/02576). Human antibody FRs ligated via the CDR are selected where the CDR forms a favorable antigen-binding site. As necessary, amino acids in the framework region of an antibody variable region may be substituted such that the CDR of a reshaped human antibody forms an appropriate antigen-binding site (Sato, K. et al., Cancer Res. (1993) 53, 851-856). Methods for obtaining human antibodies are also known. For example, desired human antibodies with antigen-binding activity can be obtained by (1) sensitizing human lymphocytes with antigens of interest or cells expressing antigens of interest in vitro; and (2) fusing the sensitized lymphocytes with human myeloma cells such as U266 (see Japanese Patent Application Kokoku Publication No. (JP-B) H01-59878 (examined, approved Japanese patent application published for opposition)). Alternatively, the desired human antibodies can also be obtained by using antigens of interest to immunize transgenic (Tg) animals comprising a partial or entire repertoire of human antibody genes (see International Patent Application WO 93/12227, WO 92/03918, WO 94/02602, WO 94/25585, WO 96/34096, and WO 96/33735). Furthermore, techniques to obtain human antibodies by panning with a human antibody library are known. For example, the variable regions of human antibodies are expressed as single chain antibodies (scFvs) on the surface of phages, using a phage display method, and the phages that bind to the antigen can be selected. By analyzing the genes of selected phages, the DNA sequences encoding the variable regions of human antibodies that bind to the antigen can be determined. If the DNA sequences of scFvs that bind to the antigen are identified, appropriate expression vectors comprising these sequences can be constructed to obtain human antibodies. Such methods are already well known (see WO 92/01047, WO 92/20791, WO 93/06213, WO 93/11236, WO 93/19172, WO 95/01438, and WO 95/15388). When the antibody genes are isolated and introduced into appropriate hosts to produce antibodies, hosts and expression vectors can be used in appropriate combinations. Eukaryotic host cells that can be used are animal cells, plant cells, and fungal cells. The animal cells include: (1) mammalian cells such as CHO, COS, myeloma, baby hamster kidney (BHK), HeLa, and Vero cells; (2) amphibian cells such as Xenopus oocytes; or (3) insect cells such as sf9, sf2l, and Tn5. Known plant cells include cells derived from the Nicotiana genus such as Nicotiana tabacum, which can be callus-cultured. Known fungal cells include yeasts such as the Saccharomyces genus, for example Saccharomyces cerevisiae, and filamentous fungi such as the Aspergillus genus, for example Aspergillus niger. Prokaryotic cells can also be used in production systems that utilize bacterial cells. Known bacterial cells include E. coli and Bacillus subtilis. The antibodies can be obtained by introducing the antibody genes of interest into these cells by transformation, and then culturing the transformed cells in vitro.

The antibodies may be minibodies or modified products of antibodies, as long as they can bind to antigens. In the present invention, a minibody comprises an antibody fragment obtained by deleting a portion from a whole antibody (for example, whole IgG). There is no limitation on the type of minibody, as long as it has the ability to bind to an antigen. The antibody fragments of the present invention are not particularly limited, as long as they are portions of whole antibodies. The antibody fragments preferably comprise a heavy chain variable region (VH) or a light chain variable region (VL), and particularly preferably comprise both a VH and VL. Specifically, the antibody fragments include Fab, Fab′, F(ab′)₂, Fv, and scFv (single-chain Fv). A preferred antibody fragment is scFv (Huston, J. S. et al., Proc. Natl. Acad. Sci. U.S.A. (1988) 85, 5879-5883; and Plickthun “The Pharmacology of Monoclonal Antibodies” Vol. 113, Eds. Resenburg and Moore, Springer Verlag, New York, pp. 269-315, (1994)). Such an antibody fragment can be prepared by treating an antibody with an enzyme (for example, papain or pepsin) or by inserting a gene construct encoding the antibody fragment into an expression vector and expressing it in appropriate host cells (see, for example, Co, M. S. et al., J. Immunol. (1994) 152, 2968-2976; Better, M. and Horwitz, A. H., Methods Enzymol. (1989) 178, 476-496; Pluckthun, A. and Skerra, A., Methods Enzymol. (1989) 178, 497-515; Lamoyi, E., Methods Enzymol. (1986) 121, 652-663; Rousseaux, J. et al., Methods Enzymol. (1986) 121, 663-669; and Bird, R. E. and Walker, B. W., Trends Biotechnol. (1991) 9, 132-137). The minibodies of the present invention preferably have a smaller molecular weight than whole antibodies. However, the minibodies may form multimers (for example, dimers, trimers, or tetramers), and thus their molecular weights can be greater than those of whole antibodies.

Preferred minibodies of the present invention comprise two or more antibody VHs and two or more antibody VLs, in which each of the variable regions are directly linked, or indirectly linked together via linkers or such. The linkages may be covalent or non-covalent bonds, or comprise both covalent and non-covalent bonds. More preferred minibodies are antibodies comprising two or more VH-VL pairs formed via non-covalent bonding between VH and VL. The distance between the two VH-VL pairs in a minibody is preferably less than that in the whole antibody.

Particularly preferred minibodies of the present invention are diabodies and sc(Fv)2. A diabody is a dimerized fragment in which two variable regions are linked together via a linker or such (for example, scFv) (hereinafter, referred to as “a fragment constituting a diabody”), and which typically comprises two VL and two VH (P. Holliger et al., Proc. Natl. Acad. Sci. USA, 90, 6444-6448 (1993), EP 404097; WO 93/11161; Johnson et al., Methods in Enzymology, 203, 88-98, (1991); Holliger et al., Protein Engineering, 9, 299-305, (1996); Perisic et al., Structure, 2, 1217-1226, (1994); John et al., Protein Engineering, 12(7), 597-604, (1999); Holliger et al., Proc. Natl. Acad. Sci. USA. 90, 6444-6448, (1993); Atwell et al., Mol. Immunol. 33, 1301-1312, (1996)). The links between fragments constituting a diabody may be non-covalent or covalent bonds, and are preferably non-covalent bonds.

Alternatively, two fragments constituting a diabody can be linked together via a linker to form a single-chain diabody (scDiabody). When the fragments constituting the diabody are linked together using a long linker, comprising approximately 20 amino acids, it is possible to link the fragments constituting the diabody in the same chain using a non-covalent bond, forming a dimer.

Fragments constituting diabodies include a VL and VH linked together, a VL and VL linked together, a VH and VH linked together, and the like. A VH and VL linked together is preferred. There is no limitation on the type of linker for linking a variable region and variable region in a fragment constituting a diabody. However, it is preferable to use a linker short enough to prevent formation of a non-covalent bond between variable regions in the same fragment. Those skilled in the art can appropriately select the length of such linkers; however, their length is typically 2 to 14 amino acids, preferably 3 to 9 amino acids, and particularly preferably 4 to 6 amino acids. In these cases, the linker between the VL and VH encoded by the same fragment is short, and thus no non-covalent bonds are formed between VL and VH on the same chain. Thus, a single-chain V region fragment is not formed, and the VL and VH form dimers with other fragments via non-covalent bonds. Further, based on the same principle for producing diabodies, a multimerized antibody such as a trimer or tetramer can be prepared by linking three or more fragments constituting a diabody.

The sc(Fv)2 of the present invention are single-chain minibodies produced by linking two VHs and two VLs with linkers and such (Hudson et al., 1999, J Immunol. Methods 231:177-189). sc(Fv)2 exhibit a particularly high agonistic activity compared to whole antibodies and other minibodies. sc(Fv)2 can be produced, for example, by linking scFv molecules with a linker.

In a preferable antibody, two VHs and two VLs are arranged in the order of VH, VL, VH, and VL ([VH]-linker-[VL]-linker-[VH]-linker-[VL]), beginning from the N terminus of a single-chain polypeptide.

The order of the two VHs and two VLs is not limited to the above arrangement, and they may be arranged in any order. Examples of arrangements are listed below:

-   [VL]-linker-[VH]-linker-[VH]-linker-[VL] -   [VH]-linker-[VL]-linker-[VL]-linker-[VH] -   [VH]-linker-[VH]-linker-[VL]-linker-[VL] -   [VL]-linker-[VL]-linker-[VH]-linker-[VH] -   [VL]-linker-[VH]-linker-[VL]-linker-[VH]

The linkers for linking the variable regions of an antibody can be arbitrary peptide linkers that can be introduced by genetic engineering, or synthetic linkers (for example, see Protein Engineering, 9(3), 299-305, 1996). However, peptide linkers are preferred in the present invention. There are no limitations as to the length of the peptide linkers. The length can be appropriately selected by those skilled in the art, depending on the purpose, and is typically 1 to 100 amino acids, preferably 3 to 50 amino acids, more preferably 5 to 30 amino acids, and even more preferably 12 to 18 amino acids (for example, 15 amino acids).

For example, such peptide linkers include:

Ser Gly Ser Gly Gly Ser Ser Gly Gly Gly Gly Gly Ser Ser Gly Gly Gly Gly Gly Gly Gly Ser Ser Gly Gly Gly Gly Gly Gly Gly Gly Gly Ser Ser Gly Gly Gly Gly Gly Gly Gly Gly Gly Gly Gly Ser Ser Gly Gly Gly Gly Gly Gly (Gly Gly Gly Gly Ser)_(n) (Ser Gly Gly Gly Gly)_(n) where n is an integer of one or larger. The lengths and sequences of peptide linkers can be appropriately selected by those skilled in the art, depending on the purpose.

In an embodiment of the present invention, particularly preferable sc(Fv)2 include the sc(Fv)2 below:

-   [VH]-peptide linker (15 amino acids)-[VL]-peptide linker (15 amino     acids)-[VH]-peptide linker (15 amino acids)-[VL]

Synthetic linkers (chemical cross-linking agents) include cross-linking agents routinely used to cross-link peptides; for example, N-hydroxy succinimide (NHS), disuccinimidyl suberate (DSS), bis(succinimidyl) suberate (BS³), dithiobis(succinimidyl propionate) (DSP), dithiobis(succinimidyl propionate) (DTSSP), ethylene glycol bis(succinimidyl succinate) (EGS), ethylene glycol bis(sulfosuccinimidyl succinate) (sulfo-EGS), disuccinimidyl tartrate (DST), disulfosuccinimidyl tartrate (sulfo-DST), bis[2-(succinimidoxycarbonyloxy)ethyl]sulfone (BSOCOES), and bis[2-(succinimidoxycarbonyloxy)ethyl]sulfone (sulfo-BSOCOES). These cross-linking agents are commercially available.

In general, three linkers are required to link four antibody variable regions together. The linkers to be used may be of the same or different types. In the present invention, a preferable minibody is a diabody, even more preferably, an sc(Fv)2. Such a minibody can be prepared by treating an antibody with an enzyme, for example, papain or pepsin, to generate antibody fragments, or by constructing DNAs encoding those antibody fragments and introducing them into expression vectors, followed by expression in an appropriate host cell (see, for example, Co, M. S. et al., 1994, J. Immunol. 152, 2968-2976; Better, M. and Horwitz, A. H., 1989, Methods Enzymol. 178, 476-496; Pluckthun, A. and Skerra, A., 1989, Methods Enzymol. 178, 497-515; Lamoyi, E., 1986, Methods Enzymol. 121, 652-663; Rousseaux, J. et al., 1986, Methods Enzymol. 121, 663-669; Bird, R. E. and Walker, B. W., 1991, Trends Biotechnol. 9, 132-137).

Antibodies with extremely high agonistic activity can be prepared by converting whole antibodies into minibodies.

Modified antibodies for use include antibodies linked to various molecules, such as polyethylene glycol (PEG). Alternatively, it is also possible to link an antibody to a radioisotope, chemotherapeutic agent, or cytotoxic substance such as a bacterial toxin. Such modified antibodies can be prepared by chemically modifying an obtained antibody. Methods for modifying antibodies have been previously established in the art.

Further, antibodies for use in the present invention may be bispecific antibodies. A bispecific antibody may comprise two antigen-binding sites that each recognizes different epitopes on a certain molecule. Alternatively, one of the antigen-binding sites may recognize a certain molecule, and the other may recognize a radioactive substance, chemotherapeutic agent, or cytotoxic substance such as a bacterium-derived toxin. When such cytotoxic substances are used, tumor cell growth can be suppressed by directly adding the cytotoxic substance to cells that express a certain molecule, and specifically damaging the tumor cells. The bispecific antibodies can be prepared by linking pairs of H and L chains from two types of antibodies, or by fusing hybridomas which produce different monoclonal antibodies to produce a fused cell producing a bispecific antibody. Further, bispecific antibodies can be prepared using genetic engineering techniques.

Antibodies in which sugar chains have been modified can also be used in the present invention. Techniques for modifying antibody sugar chains have been previously reported (for example, WO 00/61739 and WO 02/31140).

An “antibody” of the present invention includes the antibodies described above.

The antibodies expressed or produced as described above can be purified by conventional protein purification methods. The antibodies can be separated and purified, for example, by the combined use of methods appropriately selected from affinity columns such as a protein A column, column chromatography, filtration, ultrafiltration, salting out, dialysis, and so on (Antibodies A Laboratory Manual. Ed Harlow, David Lane, Cold Spring Harbor Laboratory, 1988).

The antigen-binding activity of an antibody can be assayed by conventional methods (Antibodies A Laboratory Manual. Ed Harlow, David Lane, Cold Spring Harbor Laboratory, 1988). For example, an enzyme-linked immunosorbent assay (ELISA), enzyme immunoassay (EIA), radioimmunoassay (RIA), or immunofluorescence method can be used.

The present invention also provides methods of screening for ligands having agonistic activity to mutant receptors. In these methods, first, a test substance (test compound) is contacted with a mutant receptor. The “contact” of a test substance with a mutant receptor is typically achieved by adding the test substance to a culture medium or extract of cells expressing the mutant receptor. The methods for achieving contact are not limited to this method. When the test substance is a protein or such, the “contact” can be achieved by introducing a DNA vector expressing the protein into the cells.

In these methods, the next step comprises detecting signals from the mutant receptor. Signals can be detected by the methods described above.

Next, ligands with agonistic activity are selected based on comparison with cases when a test substance (control) is not contacted. The ligands selected in this way are expected to become therapeutic agents for treating or preventing diseases associated with receptor deficiencies or mutations.

In another embodiment, a screening method of the present invention comprises the first step of contacting a test substance with a normal receptor and measuring agonistic activity. Next, the same test substance is contacted with a mutant receptor, and agonistic activity is measured. Ligands with high agonistic activity to the mutant receptor as compared to a normal receptor are then selected.

In still another embodiment, a screening method of the present invention comprises the first step of contacting a test substance with a normal receptor and measuring the agonistic activity. Next, the same test substance is contacted with a mutant receptor and agonistic activity is measured. Ligands with agonistic activity to both the mutant receptor and normal receptor are then selected.

Measuring agonistic activity in the methods described above can be achieved as described above.

Substances (compounds) obtained by the above-described screening methods of the present invention are also comprised in the present invention.

Since the ligands of the present invention (for example, antibodies) have agonistic activity, they are expected to be effective therapeutic agents for diseases caused by the impaired response of receptors on which the ligands act. Such impaired responses are attributed to receptor deficiencies or mutations. Specifically, the present invention provides therapeutic agents comprising the above-described ligands of the present invention, which are used to treat diseases caused by the mutant receptors. Representative examples of the diseases described above are thrombocytopenia, type II diabetes mellitus, and Laron syndrome.

A preferred example of a disease of the present invention is congenital amegakaryocytic thrombocytopenia (CAMT).

When ligands of the present invention or substances (compounds) obtained by the methods for screening of the present invention are used as pharmaceutical compositions, they can be formulated by methods known to those skilled in the art. As necessary, the ligands or substances can be used orally, for example, as sugar-coated tablets, capsules, elixirs, or microcapsules, or parenterally, as injections of sterile solutions or suspensions comprising water or other pharmaceutically acceptable liquids. For example, the ligands or the substances can be formulated by appropriately combining with pharmaceutically acceptable carriers or solvents, specifically, sterile water or physiological saline, vegetable oils, emulsifiers, suspending agents, surfactants, stabilizers, flavoring agents, excipients, vehicles, preservatives, binding agents, and such, and mixing at a unit dosage and form required by accepted pharmaceutical implementations. In such formulations, the amount of the active ingredient should be within the required range.

Additives in the tablets or capsules can include, for example, binders such as gelatin, corn starch, gum tragacanth, and gum Arabic; excipients such as crystalline cellulose; swelling agents such as corn starch, gelatin, and alginic acid; lubricants such as magnesium stearate; edulcorants such as sucrose, lactose, or saccharin; and flavoring agents such as peppermint, Gaultheria adenothrix oil, and cherry. When the unit dosage form is a capsule, the above-described materials can also comprise a liquid carrier such as oil. A sterile composition for injection can be formulated using a vehicle such as distilled water used for injection, according to standard protocols.

Aqueous solutions used for injections include physiological saline and isotonic solutions comprising glucose or other adjunctive agents such as D-sorbitol, D-mannose, D-mannitol, and sodium chloride. They may also be combined with an appropriate solubilizing agent such as alcohol, specifically, ethanol, polyalcohol such as propylene glycol or polyethylene glycol, or non-ionic detergent such as polysorbate 80™ or HCO-50.

Oil solutions include sesame oils and soybean oils, and can be combined with solubilizing agents such as benzyl benzoate or benzyl alcohol. They may also be formulated with buffers, for example, phosphate buffers or sodium acetate buffers; analgesics, for example, procaine hydrochloride; stabilizers, for example, benzyl alcohol or phenol; or anti-oxidants. The prepared injections are typically aliquoted into appropriate ampules.

The administration may be carried out orally or parenterally, and preferably parenterally. Specifically, injection, intranasal administration, intrapulmonary administration, percutaneous administration, or such can be used. Injections include intravenous injections, intramuscular injections, intraperitoneal injections, and subcutaneous injections. The injection solutions can also be systemically or locally administered. The administration methods can be properly selected according to the patient's age, condition, and such. When the compounds can be encoded by DNA, the DNA can be inserted into a vector for gene therapy, and gene therapy can be carried out. The dosage may be, for example, in the range of 0.0001 to 1,000 mg/kg body weight. Alternatively, the dosage may be, for example, in the range of 0.001 to 100,000 mg/person. However, the dosage is not restricted to the values described above. The dosage and administration methods depend on a patient's weight, age, and condition, and can be appropriately selected by those skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the structure of a pCOS2-hMPLfull vector.

FIG. 2 is a diagram showing the structure of a pCOS2-hMPLfullG305C vector.

FIG. 3 is a diagram showing the structure of a pBACsurf1-HMPL-FLAG vector.

FIG. 4 is a diagram showing the agonistic activity of each of the diabodies and hTPO in pCOS2-HA-Ba/F3. The vertical axis indicates O.D.450/655 nm and the horizontal axis indicates the concentration.

FIG. 5 is a diagram showing the agonistic activity of each of the diabodies and hTPO in hMPL-Ba/F3. The vertical axis indicates O.D.450/655 nm and the horizontal axis indicates the concentration.

FIG. 6 is a diagram showing the agonistic activity of each of the diabodies and hTPO in hMPL(G305C)-Ba/F3. The vertical axis indicates O.D.450/655 nm and the horizontal axis indicates the concentration.

FIG. 7 is a diagram showing the structure of a pCOS2-hMPLfullC769T vector.

FIG. 8 is a diagram showing the structure of a pCOS2-hMPLfullC823A vector.

FIG. 9 is diagram showing the construction of a TA136 sc(Fv)2 gene.

FIG. 10 is a diagram showing the structure of a pCXND3-TA136 sc(Fv)2 vector.

FIG. 11 is a diagram showing the agonistic activity of TA136 db and TA136 sc(Fv)2 in hMPL-Ba/F3 cells. The vertical axis indicates O.D.450/655 nm and the horizontal axis indicates the concentration.

FIG. 12 is a diagram showing the agonistic activity of TA136 db and TA136 sc(Fv)2 in hMPL(G305C)-Ba/F3 cells. The vertical axis indicates O.D.450/655 nm and the horizontal axis indicates the concentration.

FIG. 13 is a diagram showing the agonistic activity of TA136 db and TA136 sc(Fv)2 in hMPL(C769T)-Ba/F3 cells. The vertical axis indicates O.D.450/655 nm and the horizontal axis indicates the concentration.

FIG. 14 is a diagram showing the agonistic activity of TA136 db and TA136 sc(Fv)2 in hMPL(C823A)-Ba/F3 cells. The vertical axis indicates O.D.450/655 nm and the horizontal axis indicates the concentration.

BEST MODE FOR CARRYING OUT THE INVENTION

Herein below, the present invention will be specifically described using Examples, however, it is not to be construed as being limited thereto.

EXAMPLE 1 Establishment of a Ba/F3 Cell Line

Several reports have described CAMT patients carrying the G305C (R102P) mutation in their thrombopoietin receptor gene. In this context, an expression vector for the thrombopoietin receptor gene carrying the G305C (RI 02P) mutation was constructed and introduced into Ba/F3 cells by the method described below. The prepared DNA fragments were: the normal thrombopoietin receptor gene (SEQ ID NO: 1) and the mutant gene in which the C at nucleotide position 305 from the initiation codon has been substituted for G (SEQ ID NO: 3). These DNA fragments were digested with the restriction enzymes EcoRI and SalI, and introduced into the EcoRI-SalI site of the animal cell expression vector pCOS2-Ha to prepare pCOS2-hMPLfull (FIG. 1) and pCOS2-hMPLfullG305C (FIG. 2).

After the plasmids pCOS2-hMPLfull, pCOS2-hMPLfullG305C, and as a negative control pCOS2-Ha were treated with PvuI, 20 μg of each plasmid was transfected into Ba/F3 cells under the conditions described below. The gene was introduced at a cell density of 1×10⁷ cells/ml in PBS using GENE PULSER II (BIO-RAD) (Gene Pulser Cuvette, 0.4 cm; 0.33 kV; 950 μF). The medium was then changed with RPMI1640 comprising 10% FBS, 1 ng/ml rmIL3 (Pepro tech), 500 μg/ml Geneticin(GIBCO), 100 unit/ml penicillin, and 100 μg/ml streptomycin to select cells. As a result, hMPL-Ba/F3, hMPL(G305C)-Ba/F3, and pCOS2-HA-Ba/F3 cell lines were obtained from the respective vectors described above.

EXAMPLE 2 Preparation of the Extracellular Domain Protein of Thrombopoietin Receptor

To prepare the antigen for producing anti-thrombopoietin receptor antibodies, a system for producing and secreting the extracellular domain of human thrombopoietin receptor using the insect cell line Sf9 was constructed as described below. A gene construct comprising FLAG tag placed downstream of the extracellular domain of human thrombopoietin receptor (Gln26-Trp491) was prepared and inserted into the PstI-SmaI site of pBACsurf-1 (Novagen), to construct pBACsurf1-hMPL-FLAG (FIG. 3). The resulting gene construct (SEQ ID NO: 5) can secrete the extracellular domain of thrombopoietin receptor using a secretory signal sequence derived from baculovirus gp64 protein. 4 μg of the vector was transfected into Sf9 cells using Bac-N-Blue Transfection Kit (Invitrogen), according to the protocol attached to the kit. After three days of culture, the culture supernatants were collected and recombinant viruses were isolated using plaque assays. Stock viral solutions were prepared, and then infected to Sf9 cells. The resulting culture supernatants were collected, and adsorbed to a Q Sepharose Fast Flow column (Pharmacia). The column was eluted with PBS comprising 500 mM NaCl and 0.01% Tween20. The eluate was adsorbed to M2 Affinity Resin (Sigma). The resins were eluted with 100 mM Glycine-HCl (pH 3.5) comprising 0.01% Tween20. Immediately after elution, the eluate was neutralized with 1M Tris-Cl (pH 8.0). The resulting solution was treated by gel filtration chromatography using Superdex 200 26/60 (PBS comprising 0.01% Tween20) to purify the protein.

EXAMPLE 3 Preparation of Anti-thrombopoietin Receptor Diabody

MRL/lpr mice were immunized seven times with the purified protein of the TPOR extracellular domain. The first immunization was carried out using 100 μg of the protein, and subsequent immunizations were each performed using 50 μg of protein. The immunized cells were fused with P3-X63-Ag8-U1 (P3U1) cells by methods commonly used to prepare hybridomas. The hybridomas that produced anti-thrombopoietin receptor antibodies were selected by ELISA assay using the purified protein (VB08B, VB45B, VB033, VB140, and VB157).

Meanwhile, Balb/c mice were immunized a total of 11 times with hMPL-Ba/F3 cells at one-week to five-month intervals. 1.0×10⁷ cells were intraperitoneally administered to the mice each time. Hybridomas were then prepared by the same method as described above. The hybridomas that produced anti-thrombopoietin receptor antibody were selected (TA136).

The cDNAs for the variable regions of the antibody H and L chains were cloned from each of the hybridomas thus prepared. The cloned cDNAs were sequenced. Based on the nucleotide sequences, genes encoding diabodies were designed with FLAG tag at their C termini (VB08B db, VB45B db, VB033 db, VB140 db, VB157 db, and TA136 db), and inserted into the expression vector pCXND3 for animal cells (pCXND3-VB08B db, pCXND3-VB45B db, pCXND3-VB033 db, pCXND3-VB140 db, pCXND3-VB157 db, pCXND3-TA136 db). Each prepared vector was introduced into COS7 cells, and the culture supernatant was collected after three days of culture. The concentration of diabody in each culture supernatant was determined by BIAcore (Pharmacia) using M2 antibody (Sigma).

EXAMPLE 4 Assay for Diabody Dependency of Ba/F3 Cell Line

pCOS2-HA-Ba/F3 cells, hMPL_Ba/F3 cells, and hMPL(G305C)-Ba/F3 cells were each diluted to 2.0×10⁵ cells/ml with medium (RPMI1640 comprising 10% FBS, 100 unit/ml penicillin, and 100 μg/ml streptomycin). The cells were aliquoted (60 μl/well) into the wells of 96-well plates. hTPO (R&D) was diluted to a final concentration of 25 μg/ml with CHO-S-SFM II, and then aliquoted into the wells (40 μl/well). Each diabody/COS7 sup (VB08B db, VB45B db, VB033 db, VB140 db, VB 157 db, and TA136 db) was diluted 1, 3, 9, 27, 81, and 243 times using CHO-S-SFM II, and then aliquoted into the wells (40 μl/well). The plates were incubated for 24 hours, and then Cell Count Reagent (nacalai tesque) was added to each well (10 μl/well). The O.D.450/655 nm of each well was measured after two hours of culture. The result showed that the responsiveness of hMPL(G305C)-Ba/F3 cells to hTPO and other agonistic antibodies was markedly decreased. However. TA1 36 db (SEQ ID NO: 7) was found to exhibit strong agonistic activity to hMPL(G305C)-Ba/F3 cells, while it exhibited weak agonistic activity to hMPL_Ba/F3 cells expressing the normal receptor (FIGS. 4 to 6).

In SEQ ID NO: 8, the amino acid sequence from positions 49 to 54 corresponds to heavy chain CDR1; from 69 to 84 corresponds to heavy chain CDR2; from 117 to 123 corresponds to heavy chain CDR3; from 163 to 174 corresponds to light chain CDR1; from 190 to 196 corresponds to light chain CDR2; and from 229 to 237 corresponds to light chain CDR3.

EXAMPLE 5 Establishment of a Ba/F3 Cell Line (2)

As in Example 1, expression vectors were constructed for each of the mutant thrombopoietin receptor genes carrying the C769T (R257C) mutation and the C823A (P275T) mutation, which were found in some CAMT patients. The constructs were introduced into Ba/F3 cells. Nucleotide T was substituted for the nucleotide C at position 769 from the initiation codon in the thrombopoietin receptor gene (SEQ ID NO: 1), to produce the gene of SEQ ID NO: 9; and nucleotide A was substituted for the nucleotide C at position 823, to produce the gene of SEQ ID NO: 11. These DNA fragments were digested with the restriction enzymes EcoRI and SalI, and introduced into the EcoRI-SalI site of the animal cell expression vector pCOS2-Ha to obtain pCOS2-hMPLfullC769T (FIG. 7) and pCOS2-hMPLfullC823A (FIG. 8), respectively.

After treating the plasmids pCOS2-hMPLfullC769T and pCOS2-hMPLfullC823A with PvuI, 20 μg of each of them was transfected into Ba/F3 cells under the conditions described below. The gene transfer was carried out by electroporation at a cell density of 1×10⁷ cells/ml in PBS using GENE PULSER II (BIO-RAD) (Gene Pulser Cuvette 0.4 cm; 0.33 kV; 950 μF). The medium was then changed to RPMI1640 comprising 10% FBS, 1 ng/ml rmIL3 (Pepro tech), 500 μg/ml Geneticin (GIBCO), 100 units/ml penicillin, and 100 μg/ml streptomycin to select cells. As a result, hMPL(C769T)-Ba/F3 and hMPL(C823A)-Ba/F3 cell lines were respectively obtained from the vectors described above.

EXAMPLE 6 Preparation of Anti-Thrombopoietin Receptor Antibody sc(FV)2

TA136 sc(Fv)2 gene was constructed using pCXND3-TA136 db described above by the procedure described below (FIG. 9).

PCR was carried out using a combination of primer A (TAGAATTCCACCATGAGAGTGCTGATTCCTTTGTGGCTGTTCACAGCCTTTCCTGGTA CCCTGTCTGATGTGCAGCTGCAGG/SEQ ID NO: 15) and primer B (TGGGTGAGAACAATTTGCGATCCGCCACCACCCGAACCACCACCACCCGAACCACC ACCACCTGAGGAGACGGTGACTGAGG/SEQ ID NO: 16); and also a combination of primer C (CAGTCACCGTCTCCTCAGGTGGTGGTGGTTCGGGTGGTGGTGGTTCGGGTGGTGGC GGATCGCAAATTGTTCTCACCCAGTC/SEQ ID NO: 17) and primer D (ATTGCGGCCGCTTATCACTTATCGTCGTCATCCTTGTAGTCTTTGATTTCCAGCTTGGT G/SEQ ID NO: 18). The resulting PCR products were combined and used as a template in another PCR using primers A and D. The resulting DNA fragment of about 800 bp was digested using the restriction enzymes EcoRI and NotI, and cloned into pBacPAK9 (CLONTECH) to prepare pBacPAK9-scTA136.

PCR was then carried out using pBacPAK9-scTA136 as a template with primer E (GATGTGCAGCTGCAGGAGTCGGGAC/SEQ ID NO: 19) and primer F (CCTGCAGCTGCACATCCGATCCACCGCCTCCCGAACCACCACCACCCGATCCACCAC CTCCTTTGATTTCCAGCTTGGTGC/SEQ ID NO: 20). The resulting DNA fragment of approximately 800 bp was cloned into the pGEM-T Easy vector (Promega).

After confirming the nucleotide sequence, the DNA was digested with the restriction enzyme PvuII. The resulting DNA fragment of approximately 800 bp was inserted into the PvuII site of pBacPAK9-scTA136 to prepare pBacPAK9-TA136 sc(Fv)2. The prepared vector was digested with the restriction enzymes EcoRI and NotI. The resulting DNA fragment of approximately 1600 bp was cloned into the expression vector pCXND3 to prepare pCXND3-TA136 sc(Fv)2 (SEQ ID NO: 13; FIG. 10).

EXAMPLE 7 Evaluation of TPO-Like Agonistic Activities of TA1 36 db and TA1 36 sc(Fv)2

The DNA constructs pCXND3-TA136 db and pCXND3-TA136 sc(Fv)2 were introduced into COS7 cells. Their respective culture supernatants were collected after three days of culture. The diabody concentrations in the prepared culture supernatants were determined by BIAcore (Pharmacia) using M2 antibody (Sigma).

hMPL-Ba/F3 cells, hMPL(G305C)-Ba/F3 cells, hMPL(C769T)-Ba/F3 cells, and hMPL(C823A)-Ba/F3 cells were each diluted to 4.0×10⁵cells/ml using medium (RPMI1640 comprising 10% FBS, 100 unit/ml penicillin, and 100 μg/ml streptomycin). The cells were aliquoted into the wells of 96 well plates (60 μl/well). 40 μl of hTPO (R&D) and the culture supernatant of the COS7 cells described above were added to each well, and the plate was incubated for 24 hours. 10 μl of Cell Count Reagent (Nacalai Tesque) was added to each well. O.D.450/655 nm was determined after incubating the plate for two hours.

The results showed that, in all three mutant thrombopoietin receptor cell lines, TA136 sc(Fv)2 exhibited much stronger agonistic activity than hTPO and TA136 db (FIGS. 12 to 14). Furthermore, TA136 db was found to show agonistic activity comparable to that of the natural ligand hTPO when converted into sc(Fv)2, although in hMPL-Ba/F3 cells expressing normal thrombopoietin receptor, TA136 db exhibited weaker activity than that of hTPO (FIG. 11).

INDUSTRIAL APPLICABILITY

The present invention provides ligands (antibodies) to treat patients with diseases caused by mutant receptors, for example, CAMT; polynucleotides encoding these antibodies; vectors comprising the polynucleotides; host cells comprising the vectors; and methods for producing the antibodies. In addition, the present invention also provides methods for gene therapy using polynucleotides that encode the antibodies. The methods of the present invention provide methods for treating various genetic diseases caused by mutations in genes that encode cell membrane proteins. Henceforth, individualized genetic diagnosis is likely to become widely available for patients. The antibody engineering techniques of the present invention enable the development of pharmaceutical agents matched to individual genotypes. 

1. A minibody that acts as an agonist of a mutant human thrombopoietin receptor, the minibody comprising two or more antibody heavy chain variable regions (VH) and two or more antibody light chain variable regions (VL), each variable region being linked to at least one other variable region via a linker, wherein the minibody is a stronger agonist of the mutant receptor than is thrombopoietin, and wherein the mutant receptor includes at least one of the mutations R102P, R257C, and P275T in SEQ ID NO:2.
 2. The minibody of claim 1, wherein the mutant human thrombopoietin receptor has less responsiveness to thrombopoietin than does the non-mutated thrombopoietin receptor.
 3. The minibody of claim 1, wherein the mutant human thrombopoietin receptor causes a disease.
 4. The minibody of claim 1, wherein the minibody is a diabody.
 5. The minibody of claim 1, wherein the minibody is a sc(Fv)2.
 6. The minibody of claim 1, wherein each variable region is linked to at least one other variable region via a peptide linker.
 7. A pharmaceutical composition for treatment of a disease caused by a mutant human thrombopoietin receptor the composition comprising: (a) a minibody that comprises two or more antibody heavy chain variable regions (VH) and two or more antibody light chain variable regions (VL), each variable region being linked to at least one other variable region via a linker, wherein the minibody is a stronger agonist of the mutant receptor than is thrombopoietin, and wherein the mutant receptor includes at least one of the mutations R102P, R257C, and P275T in SEQ ID NO:2; and (b) a pharmaceutically acceptable carrier.
 8. The pharmaceutical composition of claim 7, where the mutant human thrombopoietin receptor has less responsiveness to thrombopoietin than does the non-mutated thrombopoietin receptor.
 9. The pharmaceutical composition of claim 7, wherein the disease is congenital amegakaryocytic thrombocytopenia.
 10. The pharmaceutical composition of claim 7, wherein the minibody is a diabody.
 11. The pharmaceutical composition of claim 7, wherein the minibody is a sc(Fv)2.
 12. The pharmaceutical composition of claim 7, wherein each variable region is linked to at least one other variable region via a peptide linker.
 13. An in vitro method comprising contacting the minibody of claim 1 with a cell expressing a mutant thrombopoietin receptor such that the minibody binds to and activates the mutant thrombopoietin receptor expressed on the cell.
 14. The method of claim 13, wherein the minibody is human or humanized.
 15. The method of claim 13, wherein the minibody is a diabody.
 16. The method of claim 13, wherein the minibody is a sc(Fv)2.
 17. The method of claim 16, wherein the sc(Fv)2 is human or humanized.
 18. The method of claim 13, wherein each variable region of the minibody is linked to at least one other variable region via a peptide linker.
 19. The method of claim 13, wherein the mutant thrombopoietin receptor expressed on the cell contains one or more of the mutations R102P, R257C, and P275T in SEQ ID NO:2.
 20. The minibody of claim 1, wherein the mutant thrombopoietin receptor comprises the R102P mutation.
 21. The minibody of claim 1, wherein the mutant thrombopoietin receptor comprises the R257C mutation.
 22. The minibody of claim 1, wherein the mutant thrombopoietin receptor comprises the P275T mutation.
 23. The minibody of claim 1, wherein the minibody is human or humanized.
 24. The minibody of claim 4, wherein the minibody is human or humanized.
 25. The minibody of claim 5, wherein the minibody is human or humanized.
 26. The pharmaceutical composition of claim 7, wherein the minibody is human or humanized. 